Zero return photoelectric control system

ABSTRACT

A system controls high power devices according to ambient light levels. The system includes a latching relay, a latching relay driver, an ambient light sensor and a relay bulk power supply. The latching relay switches power on and off to the high power devices, and the latching relay driver energizes the latching relay, using power from on-periods when the latching relay is closed. The ambient light sensor controls when the latching relay driver energizes the latching relay, and the relay bulk power supply stores power from the on-periods to be used to energize the latching relay during off-periods when the latching relay is open.

FIELD OF THE INVENTION

The present invention relates to photoelectric switches generally and tohigh power photoelectric switches in particular.

BACKGROUND OF THE INVENTION

Photoelectric cells, also known as photocells or photovoltaic cells,work on the principle of the photoelectric effect, and convert lightenergy into electrical energy. Such photoelectric cells can be used in aphotoelectric switch that operates according to the ambient lightconditions at the photoelectric cell. The amount of light falling on thephotocell ‘eye,’ or lens, of the switch will determine the status of theswitch, either ‘on’ or ‘standby.’

High power photoelectric switches are used to control street lighting,turning them on when the ambient light falls below a determinedswitch-on level, and then turning them off when the ambient light levelis above a determined switch-off level.

Reference is made to FIG. 1 which is a schematic illustration of astreet light 10 controlled by a high-power photoelectric switch. Streetlight 10 has a light 102 mounted on a light pole 103, and aphotoelectric controller 105 with a photoelectric eye 106 also mountedon light pole 103 or on light 102. Light 102 is turned on and off byphotoelectric controller 105, according to ambient light level 108.

Street lighting accounts for significant energy costs of a city. Usingphotoelectric switches to control such street lighting, givesflexibility to switch street lighting on and off depending on the actualambient light levels, as well as saving energy compared to othersolutions such as time-switches. However, photoelectric controller 105consumes energy even when street light 102 is off. This is known as the‘standby power consumption.’ In general, standby energy consumption isresponsible for over 5% of the total world's electricity consumption,according to the IEA (International Energy Agency). The standby powerconsumption of current photoelectric controllers 105 is around a fewhundred milliwatts, but can reach several watts. According to IEC62301:2011—Household electrical appliances—Measurement of standbypower—Clause 4.5: measurements of less than 5 mW during standby areconsidered zero power consumption, and as such, a controller 105 with astandby power consumption of less than 5 milliwatts across all operatingvoltage and temperature ranges is defined as ‘zero standby’ controller.

SUMMARY OF THE PRESENT INVENTION

There is therefore provided, in accordance with a preferred embodimentof the present invention, a system to control high power devicesaccording to ambient light levels. The system includes a latching relay,a latching relay driver, an ambient light sensor and a relay bulk powersupply. The latching relay switches power on and off to the high powerdevices, and the latching relay driver energizes the latching relay,using power from on-periods when the latching relay is closed. Theambient light sensor controls when the latching relay driver energizesthe latching relay, and the relay bulk power supply stores power fromthe on-periods to be used to energize the latching relay duringoff-periods when the latching relay is open.

Moreover, in accordance with a preferred embodiment of the presentinvention, the latching relay driver includes a first capacitor and acomparator. The first capacitor stores power during the on-periods, andthe comparator monitors a level of power stored in the relay bulk powersupply, and stops the latching relay driver from energizing the latchingrelay until the level of power is above a first predefined thresholdvalue.

Further, in accordance with a preferred embodiment of the presentinvention, the ambient light sensor includes a light dependent resistor,and a negative temperature coefficient thermistor. The light dependentresistor changes a first resistance according to ambient light levels,and the negative temperature coefficient thermistor stabilizes operationacross a predefined temperature range by changing a second resistanceaccording to temperature.

Still further, in accordance with a preferred embodiment of the presentinvention, the system also includes a dual phase power supply thattrickle charges the relay bulk power supply during the off-periods andfast charges the relay bulk power supply during the on-periods.

Additionally, in accordance with a preferred embodiment of the presentinvention, the dual phase power supply includes a dual phase rectifierconnected to a first electrically isolated alternating current inputphase and a second electrically isolated alternating current inputphase. The dual phase rectifier converts the first electrically isolatedalternating current input phase into a first constant current directcurrent output for trickle charging, and converts the first electricallyisolated alternating current input phase and the second electricallyisolated alternating current input phase into a second constant currentdirect current output for fast charging.

Moreover, in accordance with a preferred embodiment of the presentinvention, the relay bulk power supply includes a second capacitor and athermistor. The second capacitor stores the power, and the thermistorlimits the level of power stored in the second capacitor to a secondpredefined threshold value.

Further, in accordance with a preferred embodiment of the presentinvention, the latching relay driver includes a plurality of switchesand a delay circuit. The plurality of switches and a delay circuit add adelay between the plurality of switches, and control the connection ofan output of the relay bulk power supply and a ground.

Still further, in accordance with a preferred embodiment of the presentinvention, the delay circuit includes two falling edge triggeredmonostable vibrators in series, with calibrated output pulse widths.

Moreover, in accordance with a preferred embodiment of the presentinvention, the latching relay driver is a failover-to-on circuit thatsets the latching relay to closed if a third level of power to thelatching relay driver falls below a third predefined threshold valueduring the off-periods.

Additionally, in accordance with a preferred embodiment of the presentinvention, the system is a zero-standby device.

There is also provided, in accordance with a preferred embodiment of thepresent invention, a method to control high power devices according toambient light levels. The method includes switching power on and off toa load using a latching relay, energizing the latching relay using onlypower from on-periods when the latching relay is closed, controllingwhen the energizing takes place, and storing power from the on-periodsin a relay bulk power supply to be used for energizing the latchingrelay during off-periods when the latching relay is open.

Further, in accordance with a preferred embodiment of the presentinvention, energizing includes second storing power in a first capacitorduring the on-periods, monitoring a level of power stored in the relaybulk power supply, and stopping the energizing until the level of poweris above a first predefined threshold value.

Still further, in accordance with a preferred embodiment of the presentinvention, controlling includes changing a first resistance of a lightdependent resistor according to ambient light levels, and stabilizingoperation across a predefined temperature range by changing a secondresistance of a negative temperature coefficient thermistor according totemperature.

Moreover, in accordance with a preferred embodiment of the presentinvention, storing includes trickle charging the relay bulk power supplyduring the off-periods, and fast charging the relay bulk power supplyduring the on-periods.

Additionally, in accordance with a preferred embodiment of the presentinvention, trickle charging includes converting a first electricallyisolated alternating current input phase to a dual phase power supplyinto a first constant current direct current output for the tricklecharging.

Further, in accordance with a preferred embodiment of the presentinvention, fast charging includes converting the first and a secondelectrically isolated alternating current input phases to a dual phasepower supply into a second constant current direct current output forthe fast charging.

Still further, in accordance with a preferred embodiment of the presentinvention, storing includes first storing power in a second capacitor,monitoring a level of power in the second capacitor, and limiting thelevel of power stored in the second capacitor to a second predefinedthreshold value.

Moreover, in accordance with a preferred embodiment of the presentinvention, the method includes adding a delay between a plurality ofswitches controlling connection of an output of the relay bulk powersupply and a ground.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 is a schematic illustration of a prior art street lightcontrolled by a photoelectric switch;

FIG. 2 is a schematic illustration of a zero-return photoelectricactuator system;

FIG. 3 is an exemplary circuit diagram of the zero-return photoelectricactuator system of FIG. 2 ;

FIG. 4 is an exemplary circuit diagram of the ambient light sensor ofthe zero-return photoelectric actuator system of FIG. 2 ;

FIG. 5 is an exemplary circuit diagram of the actuator power supply ofthe zero-return photoelectric actuator system of FIG. 2 ;

FIGS. 6A, 6B, and 6C are circuit diagram illustrations of the latchingrelay system of the zero-return photoelectric actuator system of FIG. 2in three states;

FIG. 7A is a timing diagram illustration detailing the regular operationof the zero-return photoelectric actuator system of FIG. 2 ;

FIG. 7B is a timing diagram illustration detailing the ‘failover-on-off’operation of the zero-return photoelectric actuator system of FIG. 2 ;

FIG. 7C is a timing diagram illustration detailing the ‘failover-on-on’operation of the zero-return photoelectric actuator system of FIG. 2 ;

FIG. 8A is a schematic illustration of the delay circuit of the latchingrelay system of FIGS. 7A, 7B, and 7C; and

FIG. 8B are timing diagrams illustration detailing the operation of thedelay circuit of FIG. 8A.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, and components have notbeen described in detail so as not to obscure the present invention.

Applicant has realized that current high power photoelectric streetlighting controllers are complex, expensive to produce, consumesignificant power during standby, and are unreliable.

Applicant has realized that the two essential functions that requirepower during standby mode are ambient light monitoring and switchactuation.

Applicant has also realized that the power required for switch actuationduring standby mode may be ‘borrowed’ from power used during on mode,thus reducing standby power consumption.

Current controller designs require complex control systems to manageswitch actuation and power circuits. Applicant has realized that acontroller circuit may be implemented using simpler logic, controlling asimpler power circuit.

Reference is made to FIG. 2 which is a schematic illustration of a zerocurrent return photocell actuator (ZRPA) system 30. ZRPA system 30comprises a ZRPA 31, a power source 32, and a load 33. Power source 32,which may be the mains power supply, may be connected to the input ofZRPA 31. Load 33, which may be a streetlight, may be connected to theoutput of ZRPA 31. ZRPA 31 may comprise a latching relay system 35, anambient light sensor 36 sensing ambient light input 39, and an actuatorpower supply 37. Latching relay system 35 further comprises a latchingrelay driver 352 and a latching relay 351. Actuator power supply 37further comprises a relay bulk power supply 351 and a dual AC powersupply 372.

When the intensity of light input 39 to ambient light sensor 36 risesabove a predetermined value X_(ON), then ambient light sensor 36 mayoutput a trigger signal V_(ALS) to latching relay driver 352. Latchingrelay driver 352 may then energize latching relay 351, by switching on acurrent I_(LR) to latching relay 351, equal to I_(LR-OFF), in order tochange ZRPA 30 from an on mode wherein latching relay 351 may be closed,to a standby mode wherein latching relay 351 may be open. When latchingrelay 351 opens, current I_(LOAD) flowing through load 33 may bedisconnected. In such a case, a street light may switch from on to off.

When the intensity of light input 39 to ambient light sensor 36 fallsbelow a predetermined value, X_(OFF), then ambient light sensor 36 maychange trigger signal V_(ALS) to V_(ALS-OFF) to latching relay driver352. Latching relay driver 352 may then energize latching relay 351, byswitching on current I_(LR) to latching relay 351, equal to I_(LR-OFF),in order to change ZRPA 30 from standby mode to on mode. When latchingrelay 351 closes, current I_(LOAD) may flow through load 33. In such acase, a street light may switch from off to on.

The time period when the state of relay system 351 may transition fromopen-to-closed, or may transition from closed-to-open, may be referredto as a ‘transitional mode’ or in a ‘transitional state.’ Incontradistinction, the time periods during which ZRPA 31 is in a stablestandby state, or stable on state, may be described as an ‘operationalmode’ or an ‘operational state.’

Stand-By Mode

As mentioned hereinabove, the two essential functions that require powerduring standby mode are ambient light monitoring by ambient light sensor36 and switching latching relay 351 from open to closed. During standbymode, the amount of current required by ambient light sensor 36 tomonitor ambient light level 39 and to trigger latching relay driver 352,may be I_(ALS). The amount of current required by latching relay driver352 to energize latching relay 351 may be I_(LR-ON). Other current usedby ZRPA 31 which may be lost to ‘system losses’ may be I_(SYSTEMLOSSES).The total input current used by ZRPA 31 during standby mode, which maybe equal to the current on the return of power supply 32, I_(AC-RET)during standby mode, and may be referred to as I_(AC-IN-STDBY), whereI_(AC-IN-STDBY) may be defined as in equation 1:

I _(AC-IN-STDBY) =I _(ALS) +I _(LR-ON) +I _(SYSTEMLOSSES)  (1)

-   -   where I_(ALS) may be the standby current required by ALS 36,        I_(LR-ON) may be the current required to switch energize        latching relay 351, and I_(SYSTEMLOSSES) may represent system        losses.

It will be appreciated that ZRPA 31 may be designed to minimize systemcurrent loss I_(SYSTEMLOSSES) to be insignificant compared to thecombination of ambient light current I_(ALS) and latching relay currentI_(LR-ON). As explained hereinbelow, Applicant has realized thatI_(LR-ON) may be ‘borrowed’ from the on mode of ZRPA system 30, hencestandby current I_(AC-IN-STDBY) may now approximate I_(ALS). It will beappreciated that through efficient ambient light sensor 36 design, andlimiting I_(ALS) to a few milliamps, ZRPA system 30 may be azero-standby device.

On-Mode

Relay bulk power supply 371 may be implemented as a bulk capacitor tostore energy to operate latching relay system 305 and ambient lightsensor 36. When ZRPA system 30 is in on mode, dual AC power supply 372may fast charge relay bulk power supply 371, where the fast-chargingcurrent I_(AC-RET) may be referred to as I_(AC-IN-ON), so that when ZRPAsystem 30 switches to standby mode, energy stored by relay bulk powersupply 371 may then be used to power latching relay driver 352 and toenergize latching relay 351. When ZRPA system 30 is in standby mode orin on mode, then relay bulk power supply 371 may constantly providepower to ambient light sensor 36. It will be appreciated that bypowering ambient light sensor 36 via relay bulk power supply 371, thecomplexity of the design of actuator power supply 37 may be reduced.When ZRPA system 30 is in standby mode, dual AC power supply 372 maycontinually trickle charge to relay bulk power supply 371 with a tricklecharge current equal to I_(AC-IN-STBY).

The time taken for dual AC power supply 372 to charge relay bulk powersupply 372 from empty to full, T_(LATCHCHARGE), may be tens of minutesup to several hours since dual AC power supply 372 may only tricklecharge relay bulk power supply 371 when ZRPA system 30 is in standbymode. As the off-to-on cycle may take place once a day, and dual ACpower supply 372 may have a 12 hour period to charge, the charging timeT_(LATCHCHARGE) may not be a limiting factor. However, manufacturers arerequired to perform mandatory testing, which may require many hundredsor thousands of power cycles to be performed in a time period notreflective of the operational environment. During such testing, acharging time of T_(LATCHCHARGE) of many hours may become a significantissue, and may limit the ability of manufacturers to perform such tests.

Applicant has realized that by using a second, higher current AC inputto dual AC power supply 372 during testing to increase supplied currentto I_(AC-IN-ON), dual AC power supply 372 may fast charge relay bulkpower supply 371 during on states. Fast charging may significantlyreduce charge time T_(LATCHCHARGE), and testing may be performed in anacceptable period of time.

ZRPA

Reference is now made to FIG. 3 which is an exemplary circuit diagram ofZRPA system 30, showing power supply 32, latching relay 351, latchingrelay driver 352, ambient light sensor 36, relay bulk power supply 371,dual AC power supply 372 and load 33. FIG. 3 additionally shows theelements of latching relay 351, which comprises an energizing coil 3512,and a latch 3513. ZRPA 31 also comprises terminals T1 thru T18.

Latching Relay

Latching relay 351 may be used to switch power supply V_(AC-IN) frompower supply 32 on terminal T2, providing a switched power supply toload 33, and also a switched second power input to dual AC power supply372. Energizer coil 3513 of latching relay 351 may be between terminalsT3 and T4, which may be connected to output terminals T5 and T6 oflatching relay driver 352. In the current embodiment of the presentinvention, latching relay 352 may be a single coil device, such that therelay may be opened and closed by a low-to-high voltage transition, or ahigh-to-low voltage transition, depending on the particular deviceselected.

There are two main latching relay topologies used in such a driver: asingle polarity energy source—wherein a positive polarity pulse and anegative polarity pulse are used to switch the latching relay between anopen state and a closed state; and, a bipolar energy source—wherein twovoltage sources are used to switch the latching relay between an openstate and a closed state. It will be appreciated that a single ormulticoil latching relay may be used for latching relay 352.

Ambient Light Sensor

Reference is now made to FIG. 4 which illustrates the circuit diagram ofambient light sensor 36 sensing ambient light input 39. Ambient lightsensor 36 comprises a comparator U1, a light dependent resistor LDR, afirst constant current source CCS₁, a negative temperature coefficientthermistor NTC, a second constant current source CCS₂, and a variable dcvoltage source VS3.

Light dependent resistor LDR and constant current source CCS₁ may beconnected to a first input of comparator U1. Negative temperaturecoefficient thermistor NTC, constant current source CCS₂, and a variabledc voltage source VS₃ may be connected to a second input of comparatorU1. Comparator U1 may be powered by a DC voltage V_(RBPS) from relaybulk power supply 371 on terminal T10. Comparator U1 may output triggervoltage V_(ALS) of ambient light sensor 36 on terminal T9 that connectsto latching relay driver 352.

Comparator U1 may compare voltage V_(LDR) on a first input of comparatorU1 and voltage V_(TCO) on a second input of comparator U1, and mayoutput a trigger voltage V_(ALS) depending on their difference. On firstinput of comparator U1, voltage V_(LDR)=Current I_(LDR)* resistanceR_(LDR), where current I_(LDR) is generated by current source CCS₁, andresistance R_(LDR) is the resistance of light dependent resistor LDR andis dependent on light level 39.

Although it is desirable that light dependent resistor LDR changeresistance R_(LDR) only depending on ambient light level 39, in reality,the resistance R_(LDR) of light dependent resistor LDR may also have atemperature dependence. Therefore temperature variations may causeV_(LDR) to change. Accordingly, the second input of comparator U1 may beconnected to voltage V_(TCO) which may vary according to temperature andthus, may be used to correct for the thermal dependence of resistor LDR.Correction voltage V_(TCO) may be calculated according to equation 2:

V _(TCO)=(I _(NTC) *R _(NTC))+V _(REF3)  (2)

-   -   where current I_(NTC) is generated by current source CCS₂,        R_(NTC) is from thermistor NTC and is dependent on ambient        temperature, and V_(REF3) is a voltage offset generated by        voltage source VS3.

The voltage drop V_(NTC) across thermistor NTC may be calculated tochange in the same direction as the temperature dependence in resistorLDR, such that when there is a positive temperature dependent voltagedrop across resistor LDR, there may be a corresponding positive voltagedrop across thermistor NTC, thus compensating for temperature effects onresistor LDR and ALS 36.

Voltage V_(REF3) may be varied to adjust the predefined ambient lightlevels X_(ON) and X_(OFF), which as mentioned hereinabove, are theambient light levels 39 that may trigger ZRPA 30 to switch on and offrespectively. It will be appreciated that hysteresis in comparator U1may be used to adjust X_(ON) and X_(OFF), such that X_(ON) and X_(OFF)may be at different light levels 39.

It will also be appreciated that the design of ambient light sensor 36as a simple analog detector may reduce complexity. Furthermore, ambientlight sensor 36 may be designed such that current I_(LDR) is low enoughsuch that ZRPA system 30 may be a zero-standby device. Light dependentresistor LDR may consume less than 1 mW, negative temperaturecoefficient thermistor NTC may consume about 10 microwatts. It will alsobe appreciated that ambient light sensor 36 may also be a low duty cycledevice. It will also be appreciated that, as a result, ambient lightsensor 36 may have ultra-low power consumption which may be less than 1milliwatt.

Actuator Power Supply

Reference is now made to FIG. 5 , which is an exemplary circuit diagramof actuator power supply 37. Dual AC power supply 372 may be adual-phase input capacitive power supply, that charges relay bulk powersupply 371 via one (in the case of trickle charging) or two (in the caseof fast charging) of the input phases.

Dual AC power supply 372 comprises a dual phase rectifier 3722 with twofixed current input phases—phase 1 from power supply 32 betweenterminals T16 and T18, and phase 2 from load 33 between terminals T17and T18. Phase 1 further comprises a resistor R1 and a capacitor C1.Phase 2 further comprises a resistor R2 and a capacitor C2.

Phase 1 may be always-on, supplying a constant trickle of currentI_(AC-IN-STBY) to dual phase rectifier 3722. Phase 2 may only be activewhen load 33 is active, providing an additional current I_(AC2) suchthat dual phase rectifier 3722 may provide fast charging currentI_(AC-RET), where I_(AC-RET) may be calculated according to equation 3:

I _(AC-RET) =I _(AC-IN-STBY) +I _(AC2)  (3)

-   -   where I_(AC-IN-STBY) may be the constant trickle of current        during standby, and I_(AC2) may be the additional charging        current added for fast charging.

Phase 1 current I_(AC-IN-STBY) may be limited to about 1 mW by capacitorC1 with a value of about 1 nF. Phase 2 current I_(AC2) may be limited toabout 10 mW by capacitor C2 with a value of about 10 nF. Capacitors C1and C2 have low enough capacitance such that, at standard powerlinefrequencies of 50 Hz or 60 Hz, they may exhibit high reactiveresistance. Capacitor C1 may exhibit an impedance of about 3k ohms, andcapacitor C2 may exhibit an impedance of about 30k ohms. Such highresistance may cause each of the inputs to act as a constant currentsink, and hence, may regulate the input current into and out of dualphase rectifier 3722. Dual phase rectifier 3722 may be susceptible todamage from high in-rush currents on both of the inputs, and therefore,resistor R1, with a value of about 1k ohms, may limit input currentI_(AC-IN-STBY), and resistor R2, with a value of about 1k ohms, maylimit input current I_(AC2). The power dissipated across resistor R1 maybe insignificant to the power through input phase 1, as currentI_(AC-IN-STBY) may be so low. The power dissipated across resistor R2may be much higher than that across resistor R1, but may beinsignificant compared to the power used by load 33. Dual phaserectifier 3722 may isolate between Phase 1 input and Phase 2 input, soas to keep the input and the output of latching relay switch 3723isolated.

Relay bulk power supply 371 comprises a capacitor C4 and a Zener diodeD1. As mentioned hereinabove, relay bulk power supply 371 may be chargedby dual AC power supply 372. When voltage V_(RBPS) in relay bulk powersupply 371 reaches V_(D1), capacitor C4 may be charged and any excesscharging current may be dissipated by Zener diode D1 as heat. In onmode, such power as is dissipated as heat in Zener diode D1 is notsignificant compared to the power dissipated through load 33. It will beappreciated that dual AC power supply 372 may be a dual-phase AC powersupply that may convert two independently isolated single phases into aDC output current. It will also be appreciated that during standby mode,the power dissipated by Zener diode D1 may become significant if it istoo high compared to the power used by ambient light sensor 36. For thisreason, the power supplied by dual AC power supply 372 in standby modemay be calibrated to be very close to the power requirement of ambientlight sensor 36. In another embodiment, charge to relay bulk powersupply 371 may be limited by an active rectifier circuit rather thanZener diode D1, in which case Zener diode D1 may be present only as asafety device in case such an active rectifier circuit fails.

Latching Relay System

Reference is now made to FIGS. 6A, 6B, and 6C which illustrate latchingrelay system 35, which comprises latching relay driver 352 connected tolatching relay 351, in three states. As shown in the circuit diagrams ofFIGS. 6A, 6B and 6C, latching relay driver 352 comprises a comparatorU2, an AND gate U3, a time delay circuit U11, a switch 1 and a switch 2.

FIG. 6A shows latching relay system 35 in an ‘uninitialized’ state, whenlatching relay 351 may be closed, and switches S1 and S2 may be open,such as before power supply 32 is initially connected to ZRPA 30. FIG.6B shows latching relay system 35 when ZRPA 30 is connected to power,and is switching from uninitialized to standby mode. In this state,switch S1 may be open, and switch S2 may be closed, causing latchingrelay 351 to be opened. FIG. 6C shows latching relay system 35 when ZRPA30 is switched from standby mode to on mode. In this state, switch S1may be closed and switch S2 may be opened, causing latching relay 351 tobe closed.

Timing Diagrams

Reference is now made to FIGS. 7A, 7B and 7C, which are timing diagramsdetailing the operation of latching relay system 35.

Timing diagram A shows voltage V_(AC-IN) into dual AC power supply 372from AC power supply 32—either on or off; timing diagram B shows currentI_(AC-RET) into dual AC power supply 372—either I_(AC-IN-STBY) (whentrickle charging relay bulk power supply 371) or I_(AC-IN-STBY)+I_(AC2)(when fast charging relay bulk power supply 371); timing diagram C showsDC voltage levels V_(RBPS) on relay bulk power supply 371—which isbetween zero and a maximum of V_(D1) (and also noting two otherreference voltage levels V_(REF+) and V_(REF−)); timing diagram D showsthe voltage level V_(TP1) at Test Point 1 (TP1)—which is between zeroand V_(D1); timing diagram E shows

the output voltage V_(CMP) of comparator U2 in dual AC power supply352—which is either a logical low or a logical high; timing diagram Fshows the output voltage V_(ALS) of ambient light sensor 36—which iseither a logical high (during the day) or a logical low (during thenight); timing diagram G shows the output of AND gate U3, Gate2, inlatching relay driver 352—which is either a logical low or a logicalhigh; timing diagram H shows the output of delay circuit U11, Gate1, inlatching relay driver 352—which is either a logical low or a logicalhigh; timing diagram I shows the state of switch S2 in latching relaydriver 352—which is either open or closed; timing diagram J shows thestate of switch S1 in latching relay driver 352—which is either open orclosed; timing diagram K shows the voltage V_(C3) across capacitor C3 inlatching relay driver 352—which is between zero and V_(D1); timingdiagram L shows the current I_(LR) through the energizing coil oflatching relay 351—which is between I_(LR-OFF) and I_(LR-ON); timingdiagram M shows the state of the output latch of latching relay351—which is either open or closed.

FIGS. 7A, 7B and 7C detail a number of time points t0 thru t18, whichwill be used to explain the operation of ZRPA 30. These time pointsbroadly define boundaries between a number of time periods. The timingdiagrams in FIGS. 7A thru 7C will now be used to explain the operationof latching relay system 35. When referring to a timing diagram, thetiming diagram reference letter is shown in brackets, for example {A}.

Operation

Prior to t0—latching relay system 35 is in the ‘uninitialized’state—such as before installation. In this state (such as duringmanufacture), latching relay 351 may be set to closed and switches S1and S2 are set to open, as they require power to be set to closed.

From t0 to t1—latching relay system 35 transitions from the‘uninitialized’ to an ‘initialized’ state when ZRPA 30 is connected topower source 32 {A} at t0. At this point, relay bulk power supply 371{C} may be fast charged by dual AC power supply 372, as both inputs todual AC power supply 372 may be connected to power 32, and currentI_(AC-RET) may equal to IA_(C-IN-STBY)+I_(AC2){B}. If voltage V_(REF)rises above voltage V_(RBPS), then the output of comparator U2, V_(CMP),{E} may become a logical false. If V_(CMP) is a logical false, then thismay indicate that there is not enough energy stored in relay bulk powersupply 371 for the operation of latching relay 351, and latching relaysystem 35 may enter a waiting state. During the waiting state, the stateof latching relay 351 may not be changed. If V_(RBPS)>=V_(REF), then theoutput of comparator U2, V_(CMP), is a logical true. When output V_(CMP)is a logical true, then this may indicate that there is enough energystored in relay bulk power supply 371 for the operation of latchingrelay 351. Dual AC power supply 372 may continue charging relay bulkpower supply 371 until V_(RBPS) reaches V_(REF+) (this is the thresholdvalue of V_(REF) when charging relay bulk power supply 371 from zero) att1. When V_(RBPS) reaches V_(REF+), then comparator U2 outputs a logicaltrue voltage V_(CMP) {E}, and latching relay system 35 may exit thewaiting state, and the system may now energize coil 3512 at which pointlatching relay system 35 is initialized.

From t1 to t2—latching relay system 35 transitions from the‘initialized’ to the ‘standby’ state—as output V_(CMP) {E} is a logicaltrue, and output V_(ALS) {F} is a logical true then AND gate U3 mayoutput a logical true {G} that results in the closure of switch S2 {I}.

From t2 to t4—latching relay system 35 is in standby mode—relay bulkpower supply 371 {C} may be trickle charged by dual AC power supply 372,as the second input to dual AC power supply 372 is removed when latchingrelay 351 {M} opens, and I_(AC-RET) drops to I_(AC-IN-STBY) {B}. Dual ACpower supply 372 may continue charging until voltage V_(RBPS) reachesV_(D1) by t3. Likewise voltage V_(C3) may also reach V_(D1) by t3.Latching relay system 35 is now in standby mode.

From t4 to t6—latching relay system 35 transitions from standby state toon state—at t4 light level 39 may fall below X_(OFF), causing triggervoltage V_(ALS) to change from a logical true to a logical false {F}.This may cause AND gate U3 {G} to output a logical low, which in turnmay cause switch S2 {I} to open, and may cause delay circuit U11 tooutput a logical high and close S1 {J}. Latching relay system 35 is nowin on mode.

From t6 to t7—latching relay system 35 is in stable on mode—voltageV_(RBPS) is maintained at V_(D1) by trickle charge from dual AC powersupply 372.

From t7 to t8—latching relay system 35 transitions from on state tostandby state—at t7, light level 39 may rise above X_(ON), causingoutput V_(ALS) to change from a logical false to a logical true {F}.This may cause AND gate U3 {G} to output a logical high, which in turnmay cause switch S2 {I} to close. Latching relay system 35 may nowswitch into standby mode in a similar manner to that explained in thetransition from t1 to t2 hereinabove.

Failover

Photoelectric street light controllers are required to switch into an onstate in the case of a power failure. This is known as ‘failover to on.’This requires controllers to have systems to ensure that the switchstatus can be changed from off to on, even in the case of a powerfailure.

Latching relay system 35 may have a fail-over mode which causes switch3512 of latching relay 351 to be ‘forced’ into the closed position inthe event of a power failure. FIG. 7B is a timing diagram detailing theoperation of latching relay system 35 during a failover during standbymode.

Prior to t9—latching relay system is in standby mode—voltage V_(RBPS){C} and voltage V_(C3) {K} are at V_(D1) as shown.

From t9 to t13—latching relay system 35 transitions from ‘standby’ to‘failover’ state—in the event of a power failure at t9, power supply 32{A} may be disconnected and trickle charging {B} of ambient light sensor36 may stop. Ambient light sensor 36 may still draw power from relaybulk power supply 371, causing voltage V_(RBPS) to fall from V_(D1).When V_(RBPS) falls below V_(REF−) (the value of V_(RBPS) during standbymode below which ZRPA 30 is deemed to be in failover mode) at t10, thenoutput V_(CMP) {E} may become a logical false, causing latching relaydriver 352 to close switch 3513 of latching relay 351, similar to thetransition between t4 and t5. The only difference is that the voltageV_(TP1), {D} will continue to track the fall in voltage V_(RBPS). Thisfailover leaves latching relay system 35 with the correct settings toallow it to reinitialize or recover when power 32 is restored.

FIG. 7C is a timing diagram illustration detailing the operation oflatching relay system 35 during a failover during on mode. It will beappreciated that prior to t14—latching relay system 35 is in on mode andduring on mode, latching relay 371 is already closed.

From t14 to t18—in contradistinction, when latching relay system 35transitions from ‘on’ to ‘failover’ state and from t18—latching relaysystem 35 is in failover mode. It will be appreciated that at time t14,latching relay system 35 is already set up to recover when power isrestored, and no further activity may be required.

It will be appreciated that V_(REF+) and V_(REF−) may be set atdifferent levels by adjusting the hysteresis of comparator U2, which mayresult in X_(ON) and X_(OFF) at different ambient light levels.

Short Avoidance

It will be appreciated that if switch S1 and switch S2 close at exactlythe same time, a short circuit will be created between the output ofrelay bulk power supply 371 and ground. Applicant has realized that byputting an asymmetrical delay (a different time delay for switch S2transitioning from on-to-off and off-to-on) between activating switch S1and activating switch S2, switch S1 and switch S2 may not be closedsimultaneously.

Reference is made to FIG. 8A which is an exemplary circuit diagram ofdelay circuit U11, comprising two falling edge triggered monostablevibrators (FETMM) U12 and U13 in series. The input of FETMM U12 is IN₁₂and the output of is Q₁₂. The input of FETMM U13 is IN₁₃ and the outputof is Q₁₃. An FETMM operates by outputting a positive pulse of apredetermined pulse width, when a high-to-low voltage is input.

Reference is now made to FIG. 9B are timing diagram illustrationsdetailing the operation of the delay circuit U11. Timing diagram N showsthe input IN₁₂ which is connected to the output of AND gate U3. Thevalue of IN₁₂ is equal to Gate2, which drives switch S1 and has a valueof either a logical high or logical low; timing diagram P shows theoutput of FETMM U12, Q₁₂—which is either a logical high or a logicallow; and timing diagram Q shows the output of FETMM U13, Q₁₃ which isequal to output Gate1—which is either a logical high or a logical low.

It will be appreciated that when output Gate 2 {N} falls, it causes apulse on the output Q₁₂ of FETMM U12 {P}. The falling edge of that pulse{P} may trigger FETMM U13 {Q} to output a second pulse on Q₁₃.

It will be appreciated that the pulse widths of both output pulses arecalculated to ensure that latching relay system 35 may operate withoutswitch S1 and switch S2 being closed at the same time, and hence mayavoid a short circuit between the output of relay bulk power supply 371V_(RBPS) and ground.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. A system to control high power devices accordingto ambient light levels, the system comprising: a latching relay toswitch power on and off to said high power devices; a latching relaydriver to energize said latching relay, using power from on-periods whensaid latching relay is closed; an ambient light sensor to control whensaid latching relay driver energizes said latching relay; and a relaybulk power supply to store power from said on-periods to be used toenergize said latching relay during off-periods when said latching relayis open.
 2. The system of claim 1 wherein said latching relay drivercomprises: a first capacitor to store power during said on-periods; anda comparator to monitor a level of power stored in said relay bulk powersupply, and to stop said latching relay driver from energizing saidlatching relay until said level of power is above a first predefinedthreshold value.
 3. The system of claim 1, said ambient light sensorcomprises: a light dependent resistor to change a first resistanceaccording to ambient light levels; and a negative temperaturecoefficient thermistor to stabilize operation across a predefinedtemperature range, wherein said negative temperature coefficientthermistor to change a second resistance according to temperature. 4.The system of claim 1 and also comprising a dual phase power supply totrickle charge said relay bulk power supply during said off-periods andto fast charge said relay bulk power supply during said on-periods. 5.The system of claim 4 and wherein said dual phase power supply comprisesa dual phase rectifier connected to a first electrically isolatedalternating current input phase and a second electrically isolatedalternating current input phase to convert said first electricallyisolated alternating current input phase into a first constant currentdirect current output for said trickle charge, and to convert said firstelectrically isolated alternating current input phase and said secondelectrically isolated alternating current input phase into a secondconstant current direct current output for said fast charge.
 6. Thesystem of claim 2 wherein said relay bulk power supply comprises: asecond capacitor to store said power; and a thermistor to limit saidlevel of power stored in said second capacitor to a second predefinedthreshold value.
 7. The system of claim 6 wherein said latching relaydriver comprises a plurality of switches and a delay circuit to add adelay between said plurality of switches, to control connection of anoutput of said relay bulk power supply and a ground.
 8. The system ofclaim 7 wherein said delay circuit comprises two falling edge triggeredmonostable vibrators in series, with calibrated output pulse widths. 9.The system of claim 2 wherein said latching relay driver is afailover-to-on circuit that sets said latching relay to closed if athird level of power to said latching relay driver falls below a thirdpredefined threshold value during said off-periods.
 10. The system ofclaim 1 wherein said system is a zero-standby device.
 11. A method tocontrol high power devices according to ambient light levels, the methodcomprising: switching power on and off to a load using a latching relay;energizing said latching relay, using only power from on-periods whensaid latching relay is closed; controlling when said energizing takesplace; and storing power from said on-periods in a relay bulk powersupply to be used for energizing said latching relay during off-periodswhen said latching relay is open.
 12. The method of claim 11 whereinsaid energizing comprises: second storing power in a first capacitorduring said on-periods; monitoring a level of power stored in said relaybulk power supply; and stopping said energizing until said level ofpower is above a first predefined threshold value.
 13. The method ofclaim 11 wherein said controlling comprises: changing a first resistanceof a light dependent resistor according to ambient light levels; andstabilizing operation across a predefined temperature range by changinga second resistance of a negative temperature coefficient thermistoraccording to temperature.
 14. The method of claim 11 wherein saidstoring comprises trickle charging said relay bulk power supply duringsaid off-periods, and fast charging said relay bulk power supply duringsaid on-periods.
 15. The method of claim 14 and wherein said tricklecharging comprises converting a first electrically isolated alternatingcurrent input phase to a dual phase power supply into a first constantcurrent direct current output for said trickle charging.
 16. The methodof claim 14 and wherein said fast charging comprises converting saidfirst and a second electrically isolated alternating current inputphases to a dual phase power supply into a second constant currentdirect current output for said fast charging.
 17. The method of claim 14wherein said storing comprises: first storing power in a secondcapacitor; monitoring a level of power in said second capacitor; andlimiting said level of power stored in said second capacitor to a secondpredefined threshold value.
 18. The method of claim 12 furthercomprising adding a delay between a plurality of switches controllingconnection of an output of said relay bulk power supply and a ground.